Spokane ADU frequently asked questions

Spokane ADU questions

How much does an ADU cost in Spokane, WA?

Cost is a range that depends on the ADU type, size, site, utilities, and finishes, and it is never a guaranteed quote. Published 2026 estimates put a new detached ADU in the Spokane area at 110,000 to 260,000 dollars, and a garage conversion at 50,000 to 120,000 dollars. See our ADU cost guide for the full breakdown by ADU type, with dated ranges and cited fee figures.

Do you need a permit for an ADU in Washington state?

Yes. An ADU is a dwelling and requires building and related permits from the jurisdiction that governs your lot. The builder you hire manages the permit process. See our Spokane ADU permits guide.

What are the disadvantages of ADUs?

An ADU is a significant investment with construction disruption, financing to arrange, and ongoing responsibilities if you rent it out, such as being a landlord and maintaining a second unit. Feasibility limits like setbacks, utilities, and septic capacity can also add cost on some lots. Weigh these against the benefits of income, flexibility, and added value.

What is the size limit for an ADU in Spokane County?

Size limits are set by your jurisdiction. Inside an urban growth area, HB 1337 requires that an ADU of at least 1,000 square feet be allowed. For a specific unincorporated county parcel, confirm the current cap with Spokane County Building and Planning. Our county rules page explains how the rules split between urban growth area and non-UGA land.

Can I build an ADU on my lot in Spokane?

Most residential lots can support some form of ADU, but it depends on your jurisdiction, lot size, setbacks, and utilities. Our feasibility guide walks through a checklist, and a builder who walks the lot confirms it for your property.

How many ADUs can I have on one lot under HB 1337?

Within an urban growth area, HB 1337 (2023) requires covered jurisdictions to allow at least two ADUs on lots that permit single-family homes (RCW 36.70A.681). The City of Spokane, for example, now allows two ADUs per lot in its residential zones. Local code carries the details, so confirm how your jurisdiction has adopted it. See our HB 1337 guide.

Is owner occupancy required for an ADU in Spokane after HB 1337?

No, not as a general rule. HB 1337 bars owner-occupancy requirements inside urban growth areas, except where a unit on the lot is used as a short-term rental. Older local rules requiring you to live on site are superseded. The current rule for each jurisdiction is on its rules page.

Do I need extra parking for an ADU in Spokane?

Parking requirements vary by jurisdiction and have been affected by HB 1337, especially near major transit. Check the current standard on your jurisdiction page rather than assuming an older rule.

How long does ADU permitting take in Spokane?

It varies with the jurisdiction, the completeness of the submittal, and review workload. A complete application from an experienced builder helps avoid the most common delays. Confirm current review times with your jurisdiction.

What is the difference between a detached ADU, an attached ADU, and a junior ADU?

A detached ADU is a standalone unit on the lot. An attached ADU shares a wall with the main home or is built as an addition. A junior ADU is a smaller unit created within the existing footprint of the house, often with a limited kitchen. See our ADU type pages for detail.

How do I finance an ADU in Washington?

Common options include home equity loans and lines of credit, cash-out refinancing, renovation loans, and construction loans. This is general information, not financial advice. See our Washington ADU financing guide and talk with a licensed lender.

What is a system development charge and does it apply to my ADU?

A system development charge is a fee some jurisdictions assess to connect to public utilities such as water and sewer. Whether one applies, and how much, depends on your jurisdiction and utilities. For the City of Spokane, the 2026 water and sewer general facilities charges are published in our ADU cost guide. For Spokane County and Spokane Valley, confirm against the jurisdiction fee schedule.

What it costs, and what happens after you send it

What does an ADU cost in the Spokane area?

detached new-build $110,000 to $260,000, garage conversion $50,000 to $120,000, prefab or modular installed $80,000 to $180,000. Add roughly $3,000 to $5,000 in permit, plan review and impact fees, which is an estimate from one source rather than a published fee schedule, so confirm it with your permitting jurisdiction.

How solid are those numbers?

Treat them as planning ranges, not prices. They come from two third-party estimator sites, ADU Zoning and ADU Wizard, both read in full on 2026-08-17, neither of them a government publication. The two land in nearly the same place on total project cost, which is why totals are shown here. They differ by roughly a factor of two on cost per square foot, so a per-square-foot rate is not a reliable way to budget an ADU here and none is quoted.

How fast will someone get back to me?

Your request goes to an independent local ADU builder, usually within about an hour during the day. The builder then contacts you to look at the property and price the project on their own schedule.

What types are covered?

Detached ADUs, attached ADUs, garage conversions and basement junior ADUs. See every type.

Only a builder standing on your lot can price your actual project.